1. Field of the Invention
The present invention relates to a wire-discharge machining apparatus. In particular, the invention relates to a wire-discharge machining apparatus which performs necessary and sufficient wire-breakage prevention operations, thereby allowing machining performance to be significantly improved.
2. Description of the Related Art
In wire-discharge machining, machining speeds are improved proportionally to an increase in machining energy. However, application of the machining energy in excess of a limit causes breakage of a wire electrode, thereby seriously reducing the machining speeds. Therefore, it has been a common practice to proceed with the machining while preventing wire breakage by limiting the energy input to the machining apparatus to below a predetermined limit.
Nevertheless, the level of the energy which causes wire breakage varies according to conditions. For instance, energy below a standard energy level tends to cause wire breakage at certain locations along the machining path such as an end surface or a step of a workpeice and a corner of the electrode path. A countermeasure commonly adopted to obviate this problem is to set a limit level of the machining energy significantly lower than a standard energy level throughout the machining process, particularly when the machining includes a condition which tends to cause wire breakage. This causes a problem in that, for example, the low limit set for the machining energy is applied not only during machining of portions where wire breakage tends to occur but also during machining of portions that are to be machined with energy of a normal level, with the result that the machining speed is unnecessarily reduced. Furthermore, indefiniteness of the low limit level of the machining energy makes it difficult to completely avoid wire breakage.
To solve these problems, for example, Japanese Unexamined Patent Application Publication No. 4-30915 proposed a method in which machining energy is reduced upon detection of a corner in an electrode path which is performed based on a sharp decrease in the number of the discharge pulses followed by a gradual increase of the same. FIG. 11 is a schematic view of one example of a discharge-machining-conditions adjustment circuit used in conventional wire-discharge machining, as proposed in the above-mentioned publication. In this figure, 10a denotes a counter, 10b denotes a frequency-to-voltage converting circuit, 10c denotes a threshold-setting circuit, and 10d denotes a comparator. FIG. 12 shows variation in the number of pulses and threshold of the pulses before and after passing through a corner in an electrode path. Referring now to FIG. 12, the solid line represents thresholds set in the threshold-setting circuit 10c, and the dotted line represents an actual number of pulses (that is, an output of the frequency-to-voltage converting circuit 10b).
In operation, the counter 10a counts discharge pulses, and the count value is converted at an interval of several milliseconds into a voltage value corresponding to the current number of pulses. The voltage value corresponding to the current number of pulses is input to one side of the comparator 10d and as well as to the threshold-setting circuit 10c. 
In the threshold-setting circuit 10c, when the input voltage sharply decreases, the number of pulses in 0.5 second is averaged by a short-time constant low-pass filter of, whereas, when the input voltage gradually increases, the number of pulses in 10 seconds is averaged by a long-time constant low-pass filter. In either case, a threshold is obtained through multiplication of the averaged filter output with a gain of 1.1 to 1.2, and the threshold thus obtained is output to the comparator 10d. When the current number of pulses is larger than the threshold, the comparator 10d generates a command signal for increasing the off time of the discharge pulses, thereby suppressing the increase of the number of discharge pulses. As a result, as shown in FIG. 12, the number of pulses is controlled so as not to exceed the threshold obtained by averaging the number of pulses over a short period when the number of discharge pulses sharply decreases and over a long period when the pulse number increases gradually.
In the conventional wire-discharge machining apparatus, breakage of wire electrode is prevented by the above-described method. However, wire-breakage frequently occur not only at the corners but also at various other non-linear portions of the electrode path, such as stepped portions where the work thickness abruptly varies and end-surface portions. The conventional method as described above can effectively be used only for the corners, leaving the problem unsolved for other portions where wire breakage is likely to take place.
Another big problem with the described method is that a detection parameter is the same as a control parameter. Insofar as a sharp decrease and a gradual increase of the discharge pulses are referenced to detect corners in the electrode path, the number of the discharge pulses is regarded as being a detection parameter. Meanwhile, with a long off-time set for a corner, since the number of the discharge pulses is directly controlled, the number of the discharge pulses serves also as a control parameter. In this case, it is not clearly known whether a decrease or an increase in the number of discharge pulses is to be attributed to the presence of a corner in the electrode path or to be understood as being the control results. This makes it difficult to precisely determine the start and the end of a corner, thus hampering adequate control at a corner of the electrode path.
To solve the problems as described above, an object of the present invention is to provide a wire-discharge machining apparatus that can sufficiently detect not only corners but also all other portions such as steps and end-surface portions of a work where wire-breakage occurs more frequently than at normal portions, and that can carry out necessary and sufficient wire-breakage prevention operations by using a control method that can discriminate between the parameter which is to be used for detection and the parameter which indicates the control results.
To achieve the above object, according to the present invention, there is provided a wire-discharge machining apparatus for machining a work by generating pulse-state discharges between a wire electrode and the work, comprising: evaluating means for measuring one of the cycle time, the frequency, and the ignition delay time of the discharges, for evaluating dispersion of the measured values, and for outputting an evaluation value for the dispersion; and control means for controlling machining conditions based on the evaluation value for the dispersion.
The arrangement may be such that the evaluating means evaluates at least one of the sample variance of the measured values, unbiased variance of the measured values, the standard deviation of the measured values, the variation coefficient of the measured values, the squared mean of the measured values, the distortion of the measured values, the kurtosis of the measured values, the mean deviation of the measured values, and the absolute values of the differences between the measured values and the mean value.
The evaluating means may comprise: means for determining the square of the mean of the measured values; means for determining the mean of the squares of the measured values; and means for determining the difference between the square of the mean of the measured values and the mean of the squares of the measured values.
The evaluating means may comprises: a function generator for producing outputs variable over two or more kinds in accordance with occurrence of a discharge; an integrator for integrating the outputs of the function generator; and means for outputting either the absolute value of the difference between the output of the integrator and the product of the integration period and the expected value of the output of the function generator, or an index which is in a monotonic relation to the absolute value.
The arrangement also may be such that the evaluating means comprises: a function generator for producing outputs variable over two or more kinds in accordance with occurrence of a discharge; an integrator for integrating the differences between the outputs of the function generator and the expected values of the outputs of the function generator; and means for outputting either the absolute value of the output of the integrator or an index which is in a monotonic relation to the absolute value.
The arrangement also my be such that the evaluating means comprises: a function generator for producing outputs variable over two or more kinds in accordance with occurrence of a discharge; an integrator for integrating the outputs of the function generator; means for dividing the period of integration into two regions of an equal length; and means for outputting the absolute value of the output of the integrator or an index having a monotonic relation to the absolute value; wherein the integrator performs the integration in opposite directions in the two regions of integration.
The arrangement also may be such that the evaluating means comprises: a function generator for producing outputs which are variable over two or more kinds in accordance with occurrence of a discharge and the expected values of which are zero; an integrator for integrating the outputs of the function generator; and means for outputting the absolute value of the output of the integrator or an index having a monotonic relation to the absolute value.
The function generator may comprise a frequency divider for producing an output inverted each time the discharge occurs.
The integrator may be a counter.
The wire-discharge machining apparatus may further comprise: a timer for measuring time intervals whereat the discharges occur; a first first-in first-out (FIFO) matrix for storing resultant values of measurement by the timer; a first register for storing the sum of the resultant values stored in the first FIFO matrix; a first adder for adding the resultant values of measurement by the timer to the value stored in the first register; a first subtracter for subtracting values of outputs of the first FIFO matrix from the value stored in the first register; a first squaring calculator for squaring contents of the first register; a second squaring calculator for squaring the resultant values of measurement by the timer; a second FIFO matrix for storing values of outputs of the second squaring calculator; a second register for storing the sum of the values stored in the second FIFO matrix; a second adder for adding values of outputs of the second squaring calculator to the value stored in the second register; a second subtracter for subtracting values of outputs of the second FIFO matrix from the value memorized in the second register; and a third subtracter for subtracting values of outputs of the second squaring calculator from contents of the second register; wherein the evaluating means uses an output of the third subtracter as the evaluation value for the dispersion.
The arrangement also may be such that the wire-discharge machining apparatus further comprises: a clock for generating clock pulses to be used as references for time measurement; a timer for outputting evaluation pulses at constant time intervals; a frequency divider for producing an output logically-inverted each time the discharge occurs; a counter which counts the clock pulses only when the output of the frequency divider represents preselected one of the two values, produces an output and concurrently, resets the count value each time the evaluation pulse is outputted; a subtracter for subtracting a constant reference value from the output of the counter; and an absolute-value circuit for outputting an absolute value of a resultant value of subtraction by the subtracter; wherein the evaluating means uses the output of the absolute-value circuit as the evaluation value for the dispersion.
The arrangement also may be such that the wire-discharge machining apparatus further comprises: a clock for generating clock pulses to be used as references for time measurement; a timer for outputting evaluation pulses at constant time intervals; a first frequency divider for producing an output logically-inverted each time the discharge occurs; a second frequency divider for producing an output logically-inverted each time the evaluation pulse is generated; a counter for performing a counting operation for the clock pulses only when the output of the first frequency divider represents preselected one of the two values, the counting operation being performed in the increasing direction only when the output of the second frequency divider represents preselected one of the two values, and in a decreasing direction when the output of the second frequency divider represents the other of the two values, and for outputting a count value and resetting the count value after performing the respective counting operations in the increasing direction and in the decreasing direction ver an equal period of time; and an absolute-value circuit for outputting an absolute value of the output of the counter; wherein the evaluating means uses the output of the absolute-value circuit as the evaluation value for the dispersion.
The arrangement also may be such that the wire-discharge machining apparatus further comprises: a clock for generating clock pulses to be used as references for time measurement; a timer for outputting evaluation pulses at constant time intervals; a frequency divider for producing an output logically-inverted each time the discharge occurs; a counter for performing a counting operation for the clock pulses in an increasing direction only when the output of the frequency divider represents preselected one of the two values, and in a decreasing direction being performed when the output of the frequency divider represents the other of the two values, and for outputting a count value and resetting the count value each time the evaluation pulse is generated; and an absolute-value circuit for outputting an absolute value of the output of the counter; wherein the evaluating means uses the output of the absolute-value circuit as the evaluation value for the dispersion.
The evaluating means may use, as the evaluation value for the dispersion, one of the mean value, the shifting mean value, and the sum of evaluation values for multiple dispersions.
The control means may perform the control so as to suppress machining energy when the evaluation value for the dispersion exceeds a predetermined reference value.
The control means also may be arranged to set a greater suppression for the machining energy which in accordance with an increase in the difference between the evaluation value and the reference value for the variation.
The control means may comprise at least one of: means for setting a long value of off-time; means for reducing orbital velocity of the wire electrode; means for setting a high electrode-position-controlling servo voltage; means for setting decreased duration of the discharge; and means for increasing impedance in a discharging circuit.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiment when the same is read in conjunction with the accompanying drawings.